1. Field of the Invention
The present invention relates to a block type dielectric filter.
2. Discussion of Background
A block type dielectric filter constituted by having a plurality of through holes that extend from one surface of a dielectric block toward the opposite surface with the surfaces, except for the one surface, being clad with a conductive material layer, is used in mobile communication devices such as car phones and cordless phones or in satellite communication. The one surface that is not clad with a conductive material layer is normally referred to as an open end surface.
Means for adjusting the resonance frequencies at the resonating portions of such a block type dielectric filter in the prior art include a method whereby the lengths of the through holes are varied, a method in which an electrode pattern is formed at the open end surface to achieve a specific capacitance between the resonating portions and the ground at a side surface, a method whereby an indented portion is provided to encompass the through holes or at an area that comes in contact with the through holes at the open end surface with this indented portion also being clad with a conductive material layer so that a specific level of capacitance is achieved between the indented portion and the ground at the side surface and the like (for instance, see Japanese Unexamined Patent Publication No. 226909/1993).
However, with the aggressive miniaturization going on at present in mobile communication devices, which constitute a vital application for this type of dielectric filter, continued miniaturization is also required of the block type dielectric filters that constitute a component thereof and it is becoming physically difficult to further vary the size of the dielectric block, to add minute electrode patterns or to form minute indented portions.
As a means for adjusting the coupling factor, which is another vital factor that affects the characteristics of the block type dielectric filter, a method featuring a groove provided at an approximately central area between adjacent through holes at the open end surface in a direction running perpendicular to the direction in which the through holes are arranged, in which the depth, the width and the like of the groove are varied for the purpose of adjustment is known (for instance, see Japanese unexamined Patent Publication No. 139901/1992).
However, when this method is employed, since the resonance frequency changes along with the coupling factor, it is not possible to adjust the resonance frequency independently of the coupling factor. Furthermore, in a standard resonating portion (.lambda./4) with this method, the length of the resonating portion can be reduced only by a quantity that corresponds to the dielectric constant.